1. Field of the Invention
This invention generally relates to plugging tubes by means of a radially expandable plug having an expander element, and specifically concerns both an apparatus and a method for plugging relatively small diameter heat exchange tubes in either a fossil fuel or nuclear steam generator.
2. Description of the Prior Art
Plugging devices for plugging the heat exchanger tubes of steam generators are known in the prior art. Often, such devices are used to seal off one or more of the U-shaped heat exchanger tubes contained within a nuclear steam generator when the walls of these tubes become degraded below acceptable limits. If such tubes are not plugged or repaired, they may crack and allow radioactive water from the primary side of the generator to leak into the non-radioactive water in the secondary side. This, in turn, could result in the radioactive contamination of the non-radioactive steam that Westinghouse-type nuclear steam generators provide to turn the turbines of the electric generators of the plant. Hence the plugging of potentially degraded heat exchanger tubes is an important maintenance operation.
The plugs used in such prior art devices generally comprise a tubular shell that is open on one end and closed at the other end, and which contains a frustoconcially shaped expander member. In one type of prior art plug, the expander element is a conical wedge shaped like a common cork used to seal a bottle, and is disposed completely within the interior of the shell with its larger circular end facing the inner surface of the closed distal end of the plug shell. Instead of being cylindrical, the interior walls of the shell are slightly tapered by increasing the thickness of the shell walls from the distal closed end to the proximal open end. When the cork-shaped wedge is forcefully pulled from the closed end toward the open end of the shell, it will radially expand the plug into sealing engagement with the inner wall of a tube by a wedging action. Such a plug design is completely described in U.S. Pat. No. 4,390,042 invented by Harvey D. Kucherer and assigned to the Westinghouse Electric Corporation. In this particular plug design, the cork-shaped expander wedge is forcefully pulled from the distal to the proximal end of the plug shell by means of a pull-rod that is connected to the expander member on one end and to a hydraulic ram on the other end.
In most instances, this particular plug design is capable of reliably and conveniently plugging the open ends of a potentially degraded U-shaped tube whose ends are surrounded by the thick steel tubesheet that divides the primary from the secondary side of the steam generator. The forceful pulling of the cork-shaped expander member along the axis of the shell not only radially expands the wall of the shell outwardly as the member is wedgingly drawn toward the proximal end of the shell, but further applies an extruding force to the metallic walls of the shell along the longitudinal axis of the tube.
In a variation of this design, an explosive charge is used in lieu of a pull-rod to move the cork-shaped wedge along the longitudinal axis of the tube shell. In such plugs, the expansion member is situated near the open end of the tubeshell, and the explosive charge is disposed between the proximal end of the shell and the top surface of the expansion member. When the charge is detonated, the cork-shaped wedge is pushed along the longitudinal axis of the shell until it abuts the closed distal end of the plug.
Unfortunately, there are certain mechanical limitations associated with these prior art plug designs that interfere with their usefulness in certain applications. For example, in plugs wherein a pull-rod is used to draw the cork-shaped wedge against the internally tapered walls, there is a limit as to the inner diameter of the tubes that such plugs can reliably seal. In nuclear steam generators utilizing heat exchanger tubes having inner diameters of approximately 0.50 inches or greater, this mechanical limitation usually poses no problem. On the other hand, for tubes whose inner diameter is less than 0.50 inches, it becomes increasingly difficult to design a pull-rod capable of withstanding the tensile force necessary to draw the cork-shaped wedge throughout the entire longitudinal axis of the tubeshell. Even when the pull-rod is formed from the strongest commercially available tool metals, such as Vascomax.RTM., it will still have a tendency to break off in small diameter plugs since its own external diameter can be no larger than the minimum internal diameter of the tapered interior of the plug shell, and since the tensile strength of any material decreases exponentially with its diameter. One way of solving this problem is to reduce the angle of both the cork-shaped wedge and the tapered walls within the plug shell. However, to obtain the same quality of seal, the plug must be lengthened. While the use of longer plugs poses no problem in tubes centrally located in the tubesheet, they are difficult if not impossible to use in the peripheral tubes of the tubesheet due to the long stroke the ram-operated pull-rod member has to make to completely pull the wedge through the plug shell.
Still another limitation of this prior art design arises from the size of the hydraulic ram that is required to apply the tensile force necessary to draw down the cork-shaped wedge. Such rams typically have a minimum diameter of about 4.5 inches. Yet, around the periphery of the tubesheet, a clearance of only one half inch exists between the tube and the bowl-shaped wall that forms the primary side of the nuclear steam generator. Hence, it is difficult to provide a ram that is powerful enough to generate the required tensile force yet compact enough to be easily manipulated in the limited space surrounding the peripheral heat exchanger tubes.
In an attempt to solve the foregoing problems the previously mentioned explosive-type plugs were developed. But while the use of explosives obviates the need for pull-rods and hydraulic rams, they proven in practice to generate at least as many problems as they solve. To minimize the amount of generator down-time necessary to complete the plugging operation, a group of explosive plugs are usually positioned and detonated simultaneously. However, the simultaneous detonation of a plurality of such plugs generates powerful mechanical shock waves that can break weakened sections of the U-shaped heat exchanger tubes that are not being plugged, thereby defeating the overall purpose of the plugging operation. These shock waves can also damage the sensitive monitoring instrumentation present on all nuclear steam generators. Additionally, the special arrangements that are necessary for the transportation of such explosively operated devices, and the necessity for licensed explosives technicians to install such plugs has made them substantially more expensive to use than plugs which are expanded by a hydraulically operated pull-rod.
Clearly, there is a need for a new type of plugging device that is capable of reliably plugging small diameter as well as large diameter heat exchanger tubes in steam generators. Ideally, such a device should be capable of installing plugs even in heat exchanger tubes of limited access, such as the tubes situated around the periphery of the tubesheet.